The present invention relates to a signal processing technique, applied to the readout of an array of detector elements, which corrects for the various offsets of the different elements of the arrays.
Two dimensional detector arrays are commonly used in conjunction with suitable optical systems to generate an image of a scene. The output from such an array is normally in the form of a multiplexed stream of analogue data and it is frequently the case that the desired output signal voltage for each element is superimposed on a larger offset voltage. There may also be responsivity and gain variations from element to element. It is therefore necessary to provide signal processing algorithms which correct for the different offset and gain characteristics of the different elements of the detector arrays. This is typically done by means of an electrical memory or storage mechanism which records fixed offset and gain correction parameters for each of the elements. These parameters are then read out sequentially in synchronism with the addressing of the elements of the array so that appropriate correction parameters may be applied to each element in turn. In certain types of detector array the offset voltages of the various elements are not fixed but vary with time so that this correction technique is inadequate. It would therefore be advantageous to provide a correction mechanism which would take account of time varying offsets.
The present invention provides a sensor comprising an array of detector elements and readout means for deriving from each of the elements a voltage signal relating to a detected phenomenon, the sensor having offset correction means operative to compensate for the difference between the mean output voltage derived from each element and a predetermined voltage, wherein the voltage signals are applied as input voltages to the offset correction means and for each element the offset correction means produces an output voltage that is derived from the input voltage and is continuously modified by subtracting from the input voltage a correcting voltage that is derived from an ongoing cumulative addition of a proportion of the difference between the output voltage for the said element and the predetermined voltage. If the proportion is kept constant for every cycle of the correction process, the difference between the output voltage and the predetermined voltage decays exponentially so that the output voltage approaches the predetermined voltage. The invention thus allows the reduction or removal of d.c. or low frequency offset voltages from higher frequency signal voltages. The invention also allows gain to be added to the signal chain in such a way as to amplify the desired signal while decreasing the unwanted offset voltages. The invention is particularly suitable for arrays of infrared detector elements. The elements are preferably pyroelectric detector elements.
In most applications the predetermined voltage will be the same for all elements but this need not necessarily be the case.
The invention also provides a method of processing detector array signals to compensate for differing offsets.
Usually the voltage signals from the detector elements are multiplexed whereby the voltage signals for different elements are processed in turn and the process is continuously repeated for all of the elements in turn.
The preferred embodiment of the invention is a digital system and as a result the decay of the difference between the output voltage and the predetermined voltage is only an approximation to an exponential decay. Thus, in the preferred embodiment of the invention, the offset correction means receives analogue signals from the readout means and includes an amplifier for providing output voltages, an analogue to digital converter for converting these output voltages to digital signals, a storage device for storing a number corresponding to each detector element, and a digital to analogue converter. The numbers in the storage device are incremented or decremented in proportion to the positive or negative difference between the output voltage and the predetermined voltage for the corresponding detector element. A proportion of each stored number is converted to an analogue value in the digital to analogue converter, and used to provide a correcting input voltage to the amplifier. It should be clear from the foregoing that the readout is now taken from the output of the offset correction means rather than directly from the detector elements.
An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: